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Teen girls who smoke risk breast cancer

July 11th, 2008

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Canadian Press
Friday, October 04, 2002

TORONTO (CP) -- People trying to keep teenage girls from taking up smoking may have a helping hand.

A team of Canadian researchers is reporting that women who begin smoking within five years of starting to menstruate run a significantly higher risk of developing breast cancer before the age of 50 than women who don't smoke.

The increased risk means if you were to compare 100,000 women who began smoking at or shortly after puberty with 100,000 women who had not, you would find an extra 1,000 cases of breast cancer among the smokers.

"It's a big effect," lead author Dr. Pierre Band said from Montreal on Thursday. Band is a senior epidemiologist with Health Canada, but he and his co-authors conducted the study while he was head of the epidemiology unit at the British Columbia Cancer Agency.

The research, which will be published Saturday in Lancet, a British medical journal, was hailed by cancer prevention advocates.

"It so clearly underlines the importance of preventing smoking among teenage girls," said Cheryl Moyer, director of cancer control programs for the Canadian Cancer Society.

While it has long been known smoking is a risk factor for lung cancer, its role or lack thereof in the development of breast cancer has been unclear.

Some studies have said it is not a risk factor. Others have said it is. Still others have suggested smoking actually decreases a woman's risk of developing breast cancer because it suppresses estrogen in the system. Many, though not all, breast cancers are fuelled by estrogen, requiring the hormone to grow and spread to other parts of the body.

What Band and his colleagues have discovered seems to explain the confusion.

They are reporting that smoking increases the risk of breast cancer if women do it early in life when their breast tissue is still not fully matured. But if they begin smoking only after they have carried a baby to term or have gone through menopause, it actually appears to protect against breast cancer.

Scientists refer to immature breast tissue as undifferentiated. Differentiation only occurs after a full-term pregnancy, or menopause, in the case of women who have no children. Band's research suggests that while differentiated breast tissue is not susceptible to environmental carcinogens, undifferentiated breast tissue is.

The director of the Canadian Breast Cancer Research Initiative was excited by the findings, saying they bring the picture on smoking and breast cancer into much sharper focus.

Marilyn Schneider noted the organization had at one point decided not to fund further studies on smoking and breast cancer because the field was so murky.

But Dr. Steven Narod -- a breast cancer expert who conducted one of the studies that found smoking may reduce the risk of breast cancer -- was not so quick to embrace the findings.

"I would be reluctant to conclude that smoking causes breast cancer in young women until I'd seen a lot more data, given that this really is flying in the face of dozens of studies which have seen less important effects," said Narod, who is chief of breast cancer research at Sunnybrook and Women's College Health Sciences Centre in Toronto.

With the exception of high dose radiation, there is "very, very, very scant" evidence that environmental carcinogens play any role in breast cancer, he said.

"Certainly it's conceivable that there may be a subgroup of women -- and I agree, I would look at the young ones who started smoking early and had early onset breast cancer -- for whom there was some elevation in risk associated with smoking," he said.

"I will accept that that possibility still remains to be explored ... What I'm saying is there's no way it can be as big an effect as what he (Band) proposes here."

Band countered that the Lancet's peer review process is a vigorous one. The journal demanded the authors reanalyse their data differently, he said. The resulting findings were the same.

Band and his colleagues based their findings on their study of about 2,100 women from British Columbia.

The women were asked to fill in a questionnaire detailing among other things if they smoked, when they started if they did, whether they'd had children and whether they'd gained weight through their adult years.

They found that in premenopausal women, the risk of breast cancer was elevated in women who had begun smoking within five years of their first period and among women who smoked and had never undergone a full-term pregnancy.

Women who were premenopausal and who took up smoking after their first full-term pregnancy were not at increased risk of breast cancer, though this subset of women -- 35 of 700 -- was small, Band admitted.

In post-menopausal women cigarette smoking did not increase the risk of breast cancer, regardless of when the woman started smoking or whether she had had a child.